Waste lubricating oils include used motor oil, diesel oil, crankcase oil, and transmission oil. These waste oils contain a number of contaminants arising from both their use and from additives added prior to their use. These contaminants for the most part include calcium, barium, zinc, aluminum and phosphorus arising from detergent dispersant agents, iron from engine wear, lead and light end hydrocarbons from gasoline, and water. In order to reuse these oils, the contaminants must be removed.
Due to the high viscosity of the oil, the fine colloidal or dissolved form of the contaminants and the dispersing nature of the additives, the contaminants cannot be removed by a simple filtration procedure without some preliminary treatment.
Conventionally, waste oils are treated in a process which includes metals removal steps, which steps involve coagulating or precipitating the contaminants and thereafter either filtering the oil or removing the metals in an aqueous phase. Often the oil is then clay contacted to remove further color bodies or metals remaining after the initial metals removal.
One of the most widely used metal removal steps is the acid-clay process. In this process, the light ends are first removed from the oil by steam stripping at a temperature in the range of about 500.degree.-650.degree. F. The oil is then contacted with high strength sulphuric acid to precipitate contaminants into the aqueous phase; the contaminants are then removed as an acid sludge. The remaining acidic oil product is thereafter contacted with clay at a temperature in the range of about 300.degree.-600.degree. F. to absorb additional contaminants and color bodies.
There are a number of problems associated with this acid-clay process. Firstly, the process produces large amounts of acid sludge which must be disposed of. Secondly, large volumes of corrosive acid are consumed by the process. Thirdly, up to 20% of the original waste oil is lost with the acid sludge.
Heretofore, the standard practice in this art has involved removing the majority of the contaminants (as by acid contacting), prior to contacting the oil with clay, since it is commonly accepted that the detergent dispersant agents present in the oil rendered the clay-oil mixture unfilterable. Further, high temperature treatments have been avoided when reprocessing waste lubricating oil, to avoid undue cracking of the oil.